Conventional photosensitive lithographic printing plate precursors comprise a support having a hydrophilic surface, such as an aluminum plate, having thereon a non-silver halide photosensitive layer, such as a photosensitive layer comprising an o-quinonediazide compound, and a photographic layer comprising a silver halide emulsion. Such a photosensitive lithographic printing precursor is described in, for example, U.S. Pat. No. 4,283,478.
The conventional plate making process for producing lithographic printing plates from such photosensitive lithographic printing plate precursors comprises four principal steps. First, the photographic layer is imagewise exposed to light and developed to form a silver image in the photographic layer. In the second step, the plate precursor is exposed to UV radiation, wherein the silver image formed in the photographic layer partially masks radiation to the underlying photosensitive layer, therein exposing only a portion of the photosensitive layer. In the third step, the photographic layer, which contains the silver image, is removed. Lastly, in the fourth step, the photosensitive layer is developed.
In this plate making process, it is necessary for the photographic layer, containing the silver image, to be firmly adhered to the photosensitive layer during the second step. In step two, the silver image formed in the photographic layer masks a portion of the underlying photosensitive layer as it is exposed to UV radiation. Following this second exposure, it is necessary to easily remove the photographic layer from the photosensitive layer, as recited in step three.
Considerable effort has been applied in the industry towards the development of lithographic printing plates that provide sufficient adherence of the photographic layer to the photosensitive layer during the course of development. Positive-acting non-silver photosensitive lithographic printing plates providing high pre-development image visibility by including phenolic-novolak resin are described in U.S. Pat. Nos. 4,350,753; 4,348,471; and 4,283,478. U.S. Pat. No. 3,635,709 teaches a positive-working lithographic printing plate wherein the photographic layer contains an ester of 2-diazo-1-naphthol-4(or 5) sulfonic acid with a polyhydroxy phenol. U.S. Pat. No. 3,567,445 describes a presensitized lithographic plate with two differentially spectrally sensitized layers separated by a novolak resin. The base layer is of the type used in the negative-working presensitized lithographic plate and the top layer is a photographic layer.
U.S. Pat. Nos. 5,512,420 and 5,609,981 teach processes for producing waterless lithographic printing plates comprising an aluminum substrate with a first photopolymerizable layer, a second layer of silicone rubber, a third protective film layer, and a fourth photographic layer consisting of a laser scanner silver halide emulsion with polymeric binder. The composition of the plate lends itself to the integration and implementation of digital imaging devices for printable image formation on the plate. U.S. Pat. No. 5,330,875 discloses a process for producing negative and positive original images on a bilevel printing plate containing a photosensitive layer and a silver halide overlayer. The plate utilizes a contact speed (positive or negative) layer and a camera speed (negative) overlayer in a single development process to produce a temporary image mask which is exposed sequentially to either the negative or positive original, with the resultant image subsequently developable in a single process. The camera speed overlayer comprises an Argon laser scanner sensitive silver halide emulsion coating to form a temporary mask.
While advances have been made in the manufacture of printing plate precursors, a serious problem continues to be encountered in daily operation of the plate making for the press. When the disclosed plates are subjected to cut or trim, the photographic layer is frequently damaged at the edge by mechanical shearing applied by a cutting knife. This results in the peeling off and picking off of the photographic layer from the photosensitive layer. Such mechanical damage induces imperfect images due to imperfect masking from the damaged photographic layer.
Therefore, there remains a need for improved adhesion of the photographic layer to the photosensitive layer. In addition, high resistance of the dry photographic layer to mechanical shearing, due to the cutting or trimming of the plate, is highly demanded; while at the same time, adequate wet adhesion of the photographic layer to the photosensitive layer has to be attained in order to eliminate imperfection or damage of the photographic layer due to peel-off or pick-off of the photographic layer in the course of photographic development.